Laser processing is a high and new technology developed rapidly not only for punching, marking, cutting, welding and heat treating, but also for fine machining. Laser processing, so to speak, has been applied in almost all industries, and has shown unique advantages, particularly in non-ferrous metal material processing. Lasers are the core elements of a laser processing system, and largely determine the quality of the laser processing.
At present, lasers used for laser processing include CO2 lasers, Nd:YAG solid lasers, semiconductor lasers, optical fiber lasers and so on. CO2 lasers occupy over 85% market share in the cutting field, they emit laser light with a wavelength of 10.6 μm. Nd:YAG solid lasers emit laser light with a wavelength of 1 μm, which is shorter than that of CO2 lasers, thus the laser light is easier to be absorbed by common metal materials. However, Nd:YAG lasers have a relatively large volume and a limited output power. Semiconductor lasers and optical fiber lasers have a promising application perspective due to their small volume, long lifetime, high efficiency high-quality light beam, simple maintenance and the ability to normally operate in harsh environments such as high temperature, high pressure, high vibration and high impact. Up to now, this type of lasers have been greatly improved in output power and beam quality. For example, optical fiber lasers have now realized a power output on the order of kilowatts.
During the laser processing of a workpiece, when a high energy laser beam reaches the surface of a workpiece with low optical absorptivity or high surface smoothness, the surface of the workpiece may reflect a large amount of laser energy and a part of reflected light might return to the laser along the original light path. In the case of a CO2 laser, the laser reflection may cause the drop of the laser power, the instability of the laser output, the change of the laser mode, and even the damage of the resonant cavity of the CO2 laser, the case of a semiconductor laser, due to the high power density of the laser beam, the reflected light returning to the light emitting chip of the semiconductor laser may generate a large amount of heat in a short time, and thus burn the chip of the semiconductor laser. In the case of an optical fiber laser, the reflected laser light may focus on the end surface of the outgoing optical fiber via the optical system to burn the fiber or even to reach the laser through the fiber, resulting in Q-switching and thus giant pulses output from the laser, all of which might impact the stability of the workpiece processing or damage the optical fiber laser.
In order to ensure the normal operation of the lasers and extend their lifetime, it is desirable to prevent a reflected light from damaging the lasers. At present, laser anti-reflection devices mainly include optical isolators utilizing the Faraday optical rotation effect. However, this type of optical isolator cannot withstand high-power laser irradiation, and can only work for low power lasers. Therefore, it is desired to have an anti-reflection device working for high power lasers (such as lasers with a power of hundreds up to a thousand Watts).
On the other hand, during processing, it is possible to make the processed surface of a workpiece relative to the output end face of the laser by an certain angle (e.g., 10 degrees or so), thereby preventing the reflected light from returning to the laser along the original path. Although this practice can prevent the reflected light from damaging a laser, it limits the application of some processes. Furthermore, a local molten pool formed by the laser on the surface of the workpiece during the processing randomly flows, thus the reflection surface keeps changing randomly, therefore the reflected light may still have a chance to return to the laser and damage it. Furthermore, a method for measuring the reflected light in real time can be used to monitor the reflected light, but it cannot really prevent the reflected light from damaging the laser after all.